Brassinosteroids have been recently recognized as a new class of plant hormones through the combination of molecular genetics and researches on biosyntheses (Yokota, Trends in Plant Sci., 2, pp. 137–143, 1997). Since the chemistry of brassinosteroids was established, biological activities of these homologues have been extensively studied, and their notable actions on plant growth have been revealed, which include elongation of stalks, growth of pollen tubes, inclination of leaves, opening of leaves, suppression of roots, activation of proton pump (Mandava and Annu. Rev. Plant Physiol. Plant Mol. Biol., 39, pp. 23–52, 1988), acceleration of ethylene production (Schlagnhaufer et al., Physiol. Plant, 61, pp. 555–558, 1984), differentiation of vessel elements (Iwasaki et al., Plant Cell Physiol., 32, pp. 1007–1014, 1991; Yamamoto et al., Plant Cell Physiol., 38, pp. 980–983, 1997), and cell extension (Azpiroz et al., Plant Cell, 10, pp. 219–230, 1998).
Furthermore, mechanisms and regulations of physiological actions of brassinosteroids have been being revealed by variety of studies on their biosynthesis (Clouse, Plant J. 10, pp. 1–8, 1996; Fujioka et al., Physiol. Plant, 100, pp. 710–715, 1997). At present, 40 or more brassinosteroids have been identified. Most of C28-brassinosteroids are common vegetable sterols, and they are considered to be biosynthesized from campesterol, which has the same carbon side chain as that of brassinolide.
Several Arabidopsis mutants which show characteristic dwarfism have been isolated, i.e., dwfl: Feldman et al., Science, 243, pp. 1351–1354, 1989; dim: Takahashi et al., Genes Dev., 9, pp. 97–107, 1995; and cbb1: Kauschmann et al., Plant J., 9, pp. 701–703, 1996. Their structural photomorphogenesis and dwarfism (cpd: Szekeres et al., Cell, 85, pp. 171–182, 1997) and de-etiolation (det2: Li et al., Science, 272, pp. 398–401, 1996; Fujioka et al., Plant Cell, 9, pp. 1951–1962, 1997) are known. The mutants have deficiencies in the brassinosteroid biosynthetic pathway. Furthermore, a dwarf mutant of Pisum sativum was recently characterized, and the mutant was reported to be a brassinosteroid deficient mutant (Nomura et al., Plant Physiol., 113, pp. 31–37, 1997). In these plants, use of brassinolide is known to negate severe dwarfism of the mutants. Although these findings suggest that roles of brassinosteroids are indispensable for growth and development of plants, an effective tool other than the analysis of mutants has been desired to elucidate physiological importance of brassinolide.
As seen in researches of gibberellin action, specific inhibitors against the biosynthesis are generally very effective tools for elucidating physiological functions of endogenous substances. Specific inhibitors against brassinosteroid biosynthesis are expected to provide a new tool for understanding the functions of brassinosteroids. Uniconazole is a potent plant growth regulator (PGR) which inhibits oxidation employed by cytochrome P-450 in the steps of the gibberellin biosynthesis from ent-kaurene to ent-kaurenoic acid. Yokota et al. observed slight reduction of the amount of endogenous castasterone as a side effect of that compound (Yokota et al., “Gibberellin”, Springer Verlag, New York, pp. 339–349, 1991). Although uniconazole in fact inhibits differentiation of vessel elements induced by brassinolide (Iwasaki et al., Plant Cell Physiol., 32, pp. 1007–1014, 1991), its inhibitory action against brassinolide is considered to be no more than an incidental action, because uniconazole essentially inhibits the gibberellin biosynthesis.
Several mutants deficient in biosynthetic enzymes are known for Arabidopsis, and their morphologic changes are unique to mutants with deficiency in the brassinosteroid biosynthesis. Therefore, the inventors of the present invention conducted intensive search for a compound inducing the morphologic changes unique to the mutants with the brassinosteroid biosynthesis deficiency to find a specific inhibitor against the brassinosteroid biosynthesis. As a result, they found that triazole compounds such as 4-(4-chlorophenyl)-2-phenyl-3-(1,2,4-triazoyl)butan-2-ol had the desired inhibitory action (Japanese Patent Unexamined Publication (Kokai) No. 2000-53657).
Meanwhile, it has been reported that genetic regulation of the brassinosteroid metabolism makes plants highly sensitive to brassinosteroids, and thus an effect of brassinosteroid administration is markedly enhanced (Neff, M. M., et al., Proc. Natl. Acad. Sci., USA, 96, pp. 15316–23, 1999). It may be possible to regulate plant growth by using this method. However, this technique has a problem that its application to an arbitrary plant at an arbitrary time is difficult. Further, it is known that plant growth can be regulated by administering brassinosteroids themselves to plants, and hence their yield and stress resistance can be enhanced. However, since brassinosteroids are expensive, their application as agricultural chemicals is difficult. It is expected that, if the brassinosteroid metabolism can be inhibited by a chemical agent, sensitivity of plants to brassinosteroids can be easily enhanced. However, no substance which inhibits the brassinosteroid metabolism has hitherto been known, and thus this method cannot be utilized.